VisList visList = theWorld.getVisibilityList();int polyID = Interact2D.getPolygonID(Interact2D.pickPolygon(visList, Interact2D.reproject2D3D(theCamera, buffer, x, y)));I assume that camera-space is all I need here, since the docs suggest reproject2D3D and not reproject2D3DWS. Even if this is right, how can I now get something useful out of it (like, say, the vertices of the returned polygon)?

You can get the (transformed) vertices from the PolygonManager by the polygon's ID. Not sure if that really helps here though. If you need the actual raw data for manipulating the polygons in a VertexController or something like that, you have create a kind of mapping between them. However, once they are compiled, your options to fiddle around with the polygons/vertices are limited anyway.

In that case, you might have a know order in which the terrain has been build. I would rather rely on that instead of extracting the lost information from the mesh again. If you have to do it that way, then you could find a polygon's adjacent polygons by comparing the vertices.

I made an AWT hack (as seen in the bottom). But now the line that's keeping me is this (level being an instance of Ground): SimpleVector pV = Interact2D.project3D2D(theCamera, buffer, level.getPolygonManager().getTransformedVertex(constructionPolyID, 0));How do I do this with the Ground class?

I'm confused now, so I took a step back and created a simple prototype of what I think this is supposed to do.

This works for software rendering or uncompiled objects only. If it should be applied to compiled objects, one would have to switch from individual textures to a texture atlas and changing uv-coordinates instead. Anyway, I hope this helps. It draws red squares on a green surface (or yellow ones if you press the button and drag the mouse). It uses it's own method to create the plane but it should work with planes from Primitives as well. But this way, the texturing can be handled more flexible.

The game is coming along nicely, but now I'm just abusing the CPU. From what I gather of what you wrote, I'm not going to have a problem with the mouse (the "if it should be applied to compiled objects, one would have to switch from individual textures..." comment was only about the painting of the polygon) in hardware. Right?